Short answer. If no father or mother survives, and the illegitimate decedent leaves only a spouse, the spouse inherits the entire estate. If the spouse survives together with brothers, sisters, nephews, or nieces of the decedent, the spouse gets one-half of the estate and those relatives share the other half.
What the law says
In default of the father or mother, an illegitimate child shall be succeeded by his or her surviving spouse who shall be entitled to the entire estate. If the widow or widower should survive with brothers and sisters, nephews and nieces, she or he shall inherit one-half of the estate, and the latter the other half.
Civil Code, Article 994 — Spouse of an Illegitimate Child. Read the full provision →
The spouse-alone scenario
Article 994 opens with a clear default: "in default of the father or mother, an illegitimate child shall be succeeded by his or her surviving spouse who shall be entitled to the entire estate." Once both parents are gone, a surviving spouse who is the only heir left in the picture takes the whole estate — there is no fractional split to work out in that scenario, and no other relative is entitled to a share by virtue of this article alone.
The scenario where siblings or their children also survive
The article changes the outcome once collateral relatives are also in the picture: "if the widow or widower should survive with brothers and sisters, nephews and nieces, she or he shall inherit one-half of the estate, and the latter the other half." So the presence of the decedent's siblings, or the children of a deceased sibling standing in as nephews or nieces, cuts the spouse's automatic full inheritance down to one-half, with the remaining half divided among that group.
Who this article covers has changed since 1950
Article 994 was written against the Civil Code's original classification of illegitimate children into separate sub-categories. The Family Code has since done away with that classification and recognizes a single class of illegitimate child. So whether this article applies to a given decedent no longer turns on which old sub-category they fell into — any person who was an illegitimate child of the deceased parents, under the law as it now stands, is covered by this article's default-of-parents rule.
What this article's fractions are, and are not, based on
The one-half and one-half split described here is a straightforward order of intestate succession among the illegitimate decedent's own heirs — the spouse on one side, the siblings and their children on the other — rather than a legitime figure computed by comparing an illegitimate child's share against a legitimate child's. Nothing in this article's own text ties those two specific fractions to that comparison, so read this page for what Article 994 itself says about spouse and sibling shares, not as a statement about legitimes generally.